This invention relates generally to the field of high pressure processing of materials, and more particularly to apparatus and methods for the encapsulation of workpieces requiring high pressure processing, and methods for performing such processing.
There is a continuing need in the modern economy for improved materials, produced more efficiently and at lower cost. One common procedure is the application of heat and pressure to a workpiece in the process known as "hot isostatic pressing" or "HIP".
HIP is a very versatile tool for the production or improvement of materials in a variety of ways. HIP is frequently used to eliminate porosity from a material, thereby producing a fully dense material with improved properties. HIP is also used to bond together dissimilar materials which are not conveniently otherwise joined into a single, integrated workpiece. Also, HIP is commonly used to compress powders into a fully dense, solid part.
We will use the term "workpiece" to denote any material, or combination of materials, to be HIP processed. As noted above, such workpieces could be a single solid material requiring densification, dissimilar materials to be bonded together, or powders requiring compaction. In the case of powders, the powders may be held in the desired final shape by an appropriate mold, by mechanical pressing, by the use of binding agents, or by several other techniques and combinations of techniques well known in the field. For economy of language, "workpiece" as used herein will encompass any such starting material for HIP.
HIP requires pressure to be applied uniformly to all exposed surfaces of the workpiece. This is in contrast to forging or other methods of pressure processing in which pressure is applied typically to one portion of a workpiece. The workpiece is typically constrained in a holder, thereby generating nonuniform pressures along different directions of the workpieces.
In HIP, pressures are typically generated by immersing the workpiece in a fluid (including in this term commonly considered gases when maintained at lower pressure); or powders not themselves adversely affected by the pressures applied during HIP; or surrounding the workpiece with a solid or glass having a low yield point. The pressure on the fluid (or powder, or solid) is increased typically by mechanical or thermal means, until the desired pressure is applied to the workpiece for the desired length of time.
Many workpieces can be immersed directly in the HIP pressure-transmitting fluid or power without the need for encapsulation in a container. Such workpieces are said to lack "open porosity". That is, all voids, spaces or manifestations of porosity in the workpiece are completely surrounded by nonporous portions of the workpiece, denying the pressure-transmitting medium acess to the porous region. When such workpieces without open porosity are immersed in a pressure-transmitting medium, application of suitable heat and pressure will typically remove remaining porosity, which is the goal of the HIP process.
Other workpieces, however, possess open porosity. When HIP is carried out on such workpieces in the absence of encapsulation, pressure-transmitting medium typically penetrates inside such sites of open porosity. Thus, under elevated pressures, such open porosity is not removed as pressure is applied to the internal surface of the porous site, tending to keep it open, as well as externally on the outside of such site. Pressure is thus balanced and open porosity will remain following HIP processing. Many commonly fabricated workpieces have open porosity, requiring encapsulation in a pressure-tight container or "can" to be fully densified by HIP processing. Powders to be consolidated into a single, dense workpiece have such open porosity and, therefore, cannot be consolidated without encapsulation. Even fully dense workpieces require encapsulation if two or more such workpieces are to be bonded together into a single, integrated, fully dense workpiece. Thus, in the practical application of HIP, pressure-tight cans are frequently required. The effective use of cans for HIP processing is the subject of the present invention.
The present invention relates to a reusable can having special features as described below. The features of this can should not be confused with features commonly occurring in the pressure vessel itself in which HIP processing is performed. HIP processing requires high pressures to be uniformly applied to a workpiece. This pressure is typically applied by compressing a "fluid" (as defined above) having the workpiece immersed therein. Many workpieces cannot "go naked" into pressurizing fluid, but must be encapsulated in a can. The pressurizing fluid (and canned or uncanned workpiece immersed therein) must be confined in a pressure vessel strong enough to withstand the high pressures of HIP processing and not significantly deform or change shape. Such pressure vessels are in variably designed to be reusable and invariable designed to have access ports through which pressure and temperature can be monitored. However, since the pressure vessel is not deformed, its rigidity and reuse are easily accomplished, and well known in the prior art. However, the can (if any) encapsulating the workpiece must deform (compress) in order for the encapsulated workpiece to "feel" the high pressure on the outside of the can. Making such a can both deformable and reusable is the subject of the present invention.
Canning for HIP processing is conventionally carried out by one of two general procedures. In one procedure, a pressure-tight material (typically a metal) is fabricated snugly around the outside of the workpiece to be HIP processed. Such a "conformal can" is completely sealed around the workpiece, typically by welding. The workpiece and can are then processed by HIP. The can is removed from around the workpiece (typically be mechanical removal or dissolving with chemicals), and the HIP-processed workpiece is removed. Often, the internal surface of the conformal can is coated by a chemical release agent, chosen to facilitate the separation of can from workpiece. Nevertheless, conformal canning is a time-consuming process, and results in the destruction of the can.
The other conventional encapsulation process is "media canning". In this process, the workpiece to be HIP processed is surrounded by a material (typically a granular material such as a silica sand, graphite, etc.) having grain sizes too large to penetrate into the workpiece's open porosity. Very viscous liquids or glasses are also used for media HIP provided the viscosity of such substances is sufficiently great to avoid significant penetration of open porosity. The workpiece and canning medium are then encapsulated into a can (which may have any convenient shape, typically cylindrical). The sealed can containing workpiece and granular canning medium is HIP processed in a conventional manner. The can is removed as in conformal canning and the fully densified workpiece removed and separated from the canning medium.
For many workpieces, residual gases surrounding the canned workpiece (either conformally or media canned) may be incorporated into the workpiece during HIP processing, harming the properties of the resulting HIP-processed workpiece. For this reason, it is common during both media and conformal canning for a vacuum to be drawn on the inside of the can just prior to final welding and sealing. This removes substantially all the gas surrounding the workpiece in the can, but does not remove those gases generated from within the workpiece itself (or the canning medium, if present) during the heat and pressure of the HIP process.
These conventional canning procedures suffer from several drawbacks, the improvement of which is the subject of the present invention.
Conventional media or conformal canning is a slow process. The need to weld under vacuum and remove the can following HIP takes significant amounts of time and labor, increasing HIP processing costs and reducing throughput.
Conventional canning destroys the can. For large workpieces, or cans designed to hold numerous workpieces, a can could cost thousands of dollars. Conventional canning reduces this can to scrap.
Quality control is always a concern in HIP processing. An expensive and lengthy HIP processing run can be completely ruined (along with the workpieces and the can) if the can is not completely pressure-tight during the HIP run. Conventional canning technology does not permit continuous monitoring of conditions inside the can during HIP processing.
Many important workpieces are made by consolidation of powders with the aid of a binding agent. The resulting partially densified or "preformed" workpiece contains in corporated binding agent which must be removed. Typically, such "debinding" processes required careful heating to volatilize and driver off the binding agent without harming the properties of the material or changing the shape of the workpiece. It is not unusual for such careful heating and debinding to require many days to complete. The present invention indicates an alternative, much faster, approach to debinding.